1. Technical Field
The present invention relates to the reclamation of .epsilon.-caprolactam from carpet waste, and more particularly to a method of purifying contaminated .epsilon.-caprolactam dissolved in water, in which the contaminated .epsilon.-caprolactam has been obtained by means of depolymerization of nylon-6 originating from carpet waste.
2. Background Information
U.S. Pat. No. 5,169,870, discloses a method in which nylon-6 from carpet made from nylon-6 fibers is depolymerized in the presence of phosphoric acid and steam. The phosphoric acid serves as a depolymerization catalyst. The carpet is traditionally composed of nylon-6 tufts and of a jute, polypropylene or latex backing. The backing may also contain fillers and the post-consumer carpet will also contain various sorts of dirt. These materials are present during depolymerization, due to the difficulty of fully separating the nylon-6 from the non-nylon-6 material. The non-nylon-6 material will result in impurities which have to be removed from the .epsilon.-caprolactam in order to obtain .epsilon.-caprolactam reusable for making nylon-6.
According to U.S. Pat. No. 5,169,870, this so-called "carpet" .epsilon.-caprolactam should be purified in the following manner. The volatile components and added steam which are released during depolymerization are condensed and fractionated to separate some non-aqueous contamination from this crude mixture containing water and .epsilon.-caprolactam. This crude water and .epsilon.-caprolactam mixture is then further purified by adding potassium permanganate as an oxidative treatment in order to oxidize the impurities which are not removed by the fractionation.
The addition of potassium permanganate is disadvantageous because solid manganese dioxide (MnO.sub.2) is produced during the purification. Solid manganese dioxide then has to be removed from the reaction by means of, for example, filtration. The removal of manganese dioxide is troublesome, particularly in a continuous process. A further disadvantage of the process disclosed in U.S. Pat. No. 5,169,870 is that manganese dioxide is a by-product of relatively little value and causes environmental pollution.
It is known per se to purify mixtures containing water and .epsilon.-caprolactam by means of hydrogenation. EP-A-411455, CS-A-160312 and J. Stresinka and J. Mokry, PetraChemia, 20 No. 5-6, 171-177 (1970), the entire contents of which are hereby incorporated by reference, describe a process for purification of .epsilon.-caprolactam by means of hydrogenation of an .epsilon.-caprolactam/water mixture. The .epsilon.-caprolactam as described in these references is, however, obtained via the Beckmann rearrangement of cyclohexanone oxime in fuming sulphonic acid (hereafter called "rearrangement" .epsilon.-caprolactam).
Hydrogenation has never been considered as a suitable method for purification of .epsilon.-caprolactam obtained by depolymerization of nylon-6 fibers. This consideration was based on the fact that .epsilon.-caprolactam obtained by depolymerization of nylon-6 fibers contains another kind of contamination due to the additives present in nylon-6 fibers (such as lubricants, anti-statics, anti-stain compounds, dye stuffs and flame retardants) as, for example, described in JP-A-52108991.
Another reason why more contamination is present in .epsilon.-caprolactam obtained by depolymerization compared to "rearrangement" .epsilon.-caprolactam is because the depolymerization reaction is, as a rule, conducted at higher temperatures compared to the temperature of the rearrangement reaction. Higher temperature results in more side reactions of the .epsilon.-caprolactam resulting in a higher degree of contamination.
In addition, "carpet" .epsilon.-caprolactam will contain even more contaminants due to the non-nylon-6 materials (additives and backing) and dirt present in the used carpets. Because of the presence of these contaminations, "carpet" .epsilon.-caprolactam has been purified with an oxidizing agent, preferable with KMnO.sub.4, a purification method described, for example, in JP-A-52108991 and JP-A-52111585, the entire contents of which are hereby incorporated by reference, for .epsilon.-caprolactam from nylon-6. This is also clear form DE-C-851195, the entire content of which is hereby incorporated by reference, which teaches that .epsilon.-caprolactam obtained from heavily contaminated nylon-6 sources will have to be purified with KMnO.sub.4. It is, therefore, surprising and unexpected that hydrogenation can be used advantageously as described below for treating "carpet" caprolactam.